<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>垂直对齐</title>
  <style type="text/css">
  .container {
    border: 1px solid #ccc;
    margin: 10px;
    padding: 10px;
    height: 200px;
  }

  .item {
    background-color: #ccc;
  }

  .container-1 {
    text-align: center;
    line-height: 200px;
    height: 200px;
  }

  .container-2 {
    position: relative;
  }

  .container-2 .item {
    width: 300px;
    height: 100px;
    /*border-radius: 50%;*/
    position: absolute;
    left: 50%;
    margin-left: -150px;
    top: 50%;
    margin-top: -50px;
  }

  .container-3 {
    position: relative;
  }

  .container-3 .item {
    width: 200px;
    height: 80px;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%); /*  */
  }

  .container-4 {
    position: relative;
  }

  .container-4 .item {
    width: 100px;
    height: 50px;
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    margin: auto;
  }
  </style>
</head>
<body>
<div class="container container-1">
  <span>一段文字</span>
</div>

<div class="container container-2">
  <div class="item">
    this is item 2
  </div>
</div>

<div class="container container-3">
  <div class="item">
    this is item 3
  </div>
</div>

<div class="container container-4">
  <div class="item">
    this is item 4
  </div>
</div>
</body>
</html>
